Radiocarbon dating

Radiocarbon dating is probably the most important and well-used dating technique in Bogology.  If well applied it can provide a chronology for a peat core that has an error range of perhaps 50 – 200 years at any given depth.  That’s an important thing to realise – using radiocarbon at least, we are never going to be able to apply an exact calendar age to a particular depth, though that is sometimes possible using tephra.

Radiocarbon is an isotope of carbon.  Isotopes are basically atoms of an element with the same number of protons but a different number of neutrons.  Carbon has three isotopes and they are known by the number of neutrons they each have; carbon-12, carbon-13 and carbon-14.  Radiocarbon is carbon-14.  99% of all carbon is carbon-12, about 1% is carbon-13 and only very trace amounts are carbon-14.  Crucially, carbon-14 is different from the other two isotopes because it is unstable, or radioactive, hence its name.

All living plants absorb radiocarbon through the atmospheric carbon dioxide used during photosynthesis.  Then, when a plant dies, it naturally stops absorbing radiocarbon.  Because radiocarbon is unstable, as soon as the plant dies it starts to decay.  Because we know the rate at which it decays (commonly referred to as its half-life, the time it take for the amount to halve, in the case of radiocarbon this is about 5500 years) we can work out the age of a sample by measuring how much is left.

This gives us the ‘radiocarbon age’ of a sample but this is not the same as the calendar age because over time the amount of carbon dioxide in the atmosphere has varied.  So we must ‘calibrate’ our radiocarbon dates against a record of that varying carbon dioxide.  Then finally we end up with a calendar age range for our sample.

This is quite an expensive process so it is never possible to date every layer of a core.  So we will tend to measure a date say every 50 cm and then use computer models to calculate the ages of the depths in between. You can read more about this process on our age-depth modelling page.

If that whole process sounds a little complicated, that’s because it is!  There are many different ways of carrying out that final modelling process so when undertaking our own research and also when considering the research of others, it is always really important to think about how the chronology was put together.